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Tire Replacement...all four Mandatory?

So here's the deal...a guy I work with has a Forester (2004 5spd X). He wanted to spread out the cost of tire replacement so he went to the tire dealership and wanted to replace only the front tires because in his rotation, the fronts had the least amount of tread.

His plan was to work into a routine where he was only replacing two tires at a time instead of a full set of four (same brand and model of tire).

The tire store tech gave him some **** and bull story (I think it's bull) about how it's an all wheel drive car - you HAVE to replace all four tires at the same time because if you unbalance the system with difference in tread depth and traction you can destroy your AWD drivetrain!?

No BS story, he should replace all four, and he should also be keeping them rotated so that they wear evenly.
If you do couple searches you'd find countless threads about peoples center differential’s being destroyed by not buying four tires, or driving to long on a spare. These cars have a center differential which splits the power between the front and back wheels. This differential also allows the wheels to spin at different speeds for short times, when the car is turning it compensates for this difference in speed for a few seconds and then goes back to not doing anything. If your tires tread is different, one set of tires is constantly turning more then the other, on a 5speed this will quickly overheat the fluid and will cost somewhere around $1500 to replace.

We had a 96 imprezza my wife bought new. within the first 10,000 miles, my wife got a flat and she changed the tire. every 10-20k miles, another tire would go and we replaced it. at one time we had 4 different makes of tires on the car with 4 different tread depths. we continued our trend of always replacing 1 tire at a time (but we started using the same tire brand) and at 90k miles (and 8 years later), the car ran like a champ.

is that mechanics story BS? no- it's based on sound reasoning as people will tell you here. can you get away with not changing all your tires at once? maybe? I sold the car so I dont know what happened. I don't know WHEN your transmission is supposed to blow up. some people say it happens on the drive back home. others say it takes 100k miles. maybe we sold our car just "at the right time" becuase in another 100 miles the transmission was going to fall off. But I think, if there was going to be a problem, it would have happened within the first year/10k miles- maybe even the first 2 years or 20k miles, and MAYBE WITHIN THE 7YRS or 80k MILES??? Still waiting for an angry phone call from the person I sold the car too. In fact I even saw the car around town. I'll have to assume everything is ok.

Subaru says tires have to be within 1/4" of tread depth (at least for my car). Thats like having two new tires and two almost bald tires.
So it all depends on how long this dude is going to keep the car. If he leases it and does not plan to keep - he can do whatever he wants with tires.

Hm Never bothered to check cause I run the same tires, but here it says "depth"http://www.cars101.c...wheels.html#faq1/4" circuference is just above 1/32" depth and can easily develop under normal wear.

That may be but I think the VC can withstand more abuse than the duty c solenoid??? My cheap tires are wearing so fast I'm well outside of spec but I'm not worried. My dad has had mismatched tires on his 92 mt for 60K with no ill effect yet. anecdotal evidence i suppose

1/4" circuference is just above 1/32" depth and can easily develop under normal wear.

the owners manual states 1/4" circumference. it was likely pulled from Subaru and misquoted. you can actually feel the 4EAT acting funny, i guess the TCU gets confused or something. if they are different enough the car will drive funny at slow speeds, you can feel the trans kind of bucking a little. install new tires and it goes away.

you can actually feel the 4EAT acting funny, i guess the TCU gets confused or something. if they are different enough the car will drive funny at slow speeds, you can feel the trans kind of bucking a little. install new tires and it goes away.

I thought the problem with having mismatched tires was that the VC or center differential overheated or got over worked because the different wheels were turning at different rates due to differing tire circumference. so wouldn't running at low speeds be "ok" while running at faster speeds just compound this problem? is this TCU thing yet ANOTHER problem?

I thought the problem with having mismatched tires was that the VC or center differential overheated or got over worked because the different wheels were turning at different rates due to differing tire circumference. so wouldn't running at low speeds be "ok" while running at faster speeds just compound this problem? is this TCU thing yet ANOTHER problem?

The TCU, along with the ECU, controls the duty solenoid in the AT. The anti-lock sensors on each wheel provide rotational data for the control units. AFAIK, all manufacturers of AWD AT cars have tire matching restrictions. Take the good along with the bad, right?

One issue that hasn't been emphasized is the TYPE of driving being done on 'mismatched' tires. I'd expect surface streets, stop&go, or especially wet weather, gravel, etc. to be very forgiving of mismatched tires because any torque building up in the drive train has lots of opportunities to relieve itself. Driving on smooth, hard, dry pavement at highway speeds for long periods of time may be more destructive.

Also, it may be possible to have the new tires 'shaved' a little to bring them to a closer circumference. Or one might run slightly less tire pressure to reduce the 'effective' radius - though of course that could lead to handling problems if taken to an extreme.

(Cliff Notes) if you don't want to gamble with your drivetrain, rotate your tires and change all 4. if you must get a single new tire, shave it to match circumference to the other 3.

With my problem, the dealer said that it was turning on pavement and other hard surfaces that causes the damage the fastest, though you can get it by highway driving too. When I asked if I could drive my car or wait untill I had it repaired, he said that if I was just going to take it on the interstate for a few miles for a few days it would be alright, but that I should avoid driving around town at all costs.

4EAT or 5MT?i don't remember which one.. but one's more forgiving then the other

if you're running different tires & sizes, how often fo you change your tranny/diff fluids? I ran around for a day w/ an odd sized tire and i got worried about the awd... :cool:

The auto is more forgiving, as it will give you lots of warning signs that its not happy. The manual, once the same sighns apear (basically toque bind) its too late. Replacing one tire at a time in 10,000 mile intervals is really a smart way of doing things. replacing one tire when the others have 50,000 miles on them bad. Ive been thinking about this a while, you can replace two tires (in thoery), but they both have to be on the same side of the car, which is not the smartest thing to do. The way the AWD dies is when the axle speed from the front axle and the rear axle do not match. The 4eat clutch pack is dersigned to live with a limited amount of slip (ie turns). With constant slip (mistmatched tires) the clucth unit is always slipping. Slipping causes heat, which clutch packs hate. The clutches chew themselves up and end up fusing. In the manual, the Viscous coupling heats up and never cools down. The VC heat makes the system transmit torque to all 4 wheels all the time. This will fry the coupling and chew up the center differential. A AWD repair can be from 800-1200 dollars. a St of tires are cheaper, considering you would have to buy tires that match after the repair in addition to the cost of the repair.

Per Subaru... 1/4" of circumference. This equals 1mm tread depth. A little more than 1/32nd. 0.040". 40 thou. (thousands of an inch). No, it is not much. Yes, it probably is a 'safe' number. If a tire has an 80" circumference, this is 0.3% Eg, one tire rotates 1000 revolutions, the other tire rotates 1003.

I have a different take on auto vs manual. One hears many more cases of torque bind with autos than with manuals (like 10 times). Just from that, I would say that the autos are more sensitive to tire matching. However, I know that some of the issues with the 96's and 97's had to do with a weakness (bushing) in the design of the auto clutch pack. So... hard to really say.

I read some years ago of a guy that had 2 VC's fail and the third one was failing. They discovered that he had 1.5" difference in circumference, front to back! Even at that, it took months (I believe) to kill the VC. It was not instantaneous.

The real point is, the tires have to match in circumference (within the tolerance). Even further, one should say "effective" or "rolling" circumference. Strictly speaking, they do not have to be replaced as a set. Circumference varies by brand and model, tread wear and inflation. So by playing with those parameters, one can find work arounds. For example, shave the new (single) tire as was mentioned. Perhaps play with pressures (but this will only get you so far). Or... you might find a new tire that matches the circumference of the old ones. Replace 2 now, then in a few months, replace the other two. That would take some doing, but it is theoretically possible.

I have a different take on auto vs manual. One hears many more cases of torque bind with autos than with manuals (like 10 times). Just from that, I would say that the autos are more sensitive to tire matching. However, I know that some of the issues with the 96's and 97's had to do with a weakness (bushing) in the design of the auto clutch pack. So... hard to really say.

Automatics do have more occurances of torquebind, but in 90% of the cases its fixed by flushing the tranny and making sure the tires match. Once you get torque bind to appear in a manual where its noticable , its usually too late, as there is no way of rectifying the situtaion. The clutchpack design was rectified in 97 1/2. Both cars are sensative to mismtached tires, just the automatic gives you lots of warning before damage occures. The manual gives you no warning that something is wrong untill you have problems making turns. There have been more total failures from mismtached tires in manuals of late then in automatics. There have been alot ofexamples of torque bind in automatics, but fewer and fewer failures of the units. Either way, matching tires are a good insurance policy, along with regular automatic fluid changes.

Automatics do have more occurances of torquebind, but in 90% of the cases its fixed by flushing the tranny and making sure the tires match. Once you get torque bind to appear in a manual where its noticable , its usually too late, as there is no way of rectifying the situtaion.The clutchpack design was rectified in 97 1/2. Both cars are sensative to mismtached tires, just the automatic gives you lots of warning before damage occures. The manual gives you no warning that something is wrong untill you have problems making turns. There have been more total failures from mismtached tires in manuals of late then in automatics. There have been alot ofexamples of torque bind in automatics, but fewer and fewer failures of the units.Either way, matching tires are a good insurance policy, along with regular automatic fluid changes.

nipper

Yeah, the auto is subject to ADDITIONAL abuse by improper maintenance - that likely is why it seems to be mentioned more often. And myabe sales of ATs is higher? dunno

keep in mind, if you do have to buy tires a little sooner than you'd like, it really isn't a total waste - you're getting NEW tires! And, with AWD, you likely would have better performance with cheap tires than a 2WD car with expensive tires. So just get 'value' tires and rotate them.

...........I have a different take on auto vs manual. One hears many more cases of torque bind with autos than with manuals (like 10 times). Just from that, I would say that the autos are more sensitive to tire matching. However, I know that some of the issues with the 96's and 97's had to do with a weakness (bushing) in the design of the auto clutch pack. So... hard to really say.

The Subaru sales of AT vs. MT are about ten to one as well. So, let's keep this thing in perspective. Plus, with the AT you have the FWD option simply by installing a fuse. I love to drive a stick myself, but it seems the manual transmission is going the same way as the manual choke.

the older generation forums, considering how old the transmissions are rarely see torque bind compared to some of the 90's models. i've never seen any torque bind in the XT6's 4EAT's i've worked on and rarely (if ever) see it on the XT boards, not sure why that changed in the 90's. they will act funny sometimes on mismatched tires. not sure what it does but the trans shuttered and jumped around at slow speeds until i replaced one bad tire. it's like the TCU was confused.